TOPIC:

Should there be an oring on the back of the carb holder? On my 550 it uses an oring the holders look similar.

The boots on my 750 had a raised lip so to speak which replicated an O-ring, but there was no discrete O-ring. From the photo it looks like these boots are similar. Sanding the boots would remove any remnants of the lip so go down that road with caution.

I'm skeptical that sealer alone will last. I'd be inclined to cut a gasket out of soft gasket paper and then use sealer on that. The gasket will confirm some when torqued. There are lots of KZ400 boots available, and the 400 used BS34 carbs on at least some models. I wonder how close those would be?

A combination of sealant and selfmade gasket paper will do the same or better Job.
The selant and paper gasget will seal/compensate perfect the gap on uneven rubber boots to the intake surface of the cylinder head.

Yamabond #4 works great and holds up for decades.

I used some red high heat Permatex, just a thin smear. It's been over 80,000 miles and not leaking yet.

I went with the red Permatex Permashield. It's says comparable to "Blue Hylomar" on the front and I had a 25% off coupon. In addition to that, I used a soft brass brush to really clean up the backs of the carb holders where they meet the head. I also replaced the screws with some nice hex socket cap screws so I could get my torque wrench in there. So far so good, really made a difference on the carbtune readings when syncing and I can actually see the carbs "dancing" a bit when starting the bike because of the vacuum in the holder. Nice. I'l update this as I get some miles on the bike as to how it's holding up.

How is this holding up?

I am going to go down the same road.

So far so good, has only been four months but I have not found any leaks as I fine tune the bike.

I used to gray Permatex that's safe for gasoline also a good for splitting cases been on my KZ1000 for almost a year no leaks no problems hope that helps

I had no idea that you were supposed to use anything between that connection. Thanks again for this great site and it's info. Knowledge = Power.!!!

Your not with new manifolds but when they get old the built in seal gets compromised so a sealant is needed. Just dont use silicone.

I wanted to update this as the repair ended up failing. The bike has been parked since last June, I recently dug it out to get tuned up for spring and after fiddling with a very inconsistent idle and poor throttle response I checked the boots for leaks. Major air leaks on both boots where they meet the cylinder head. Looks like I will be pulling them off and making gaskets or something. I am not getting any detectable leaks on the boots themselves. I will also take detailed measurements on their size and try sourcing a replacement.